This invention is in the field of coatings, in particular, water reducible coatings. For purposes of this application, a water reducible coating is defined to be a colloidal dispersion in an organic solvent and water whose resin concentration can be reduced with water or solvent, which when reduced with a coalescing solvent forms a durable coating when applied to a substrate surface. Other names applied to water reducible coatings are water-borne, water solubilized, and water dilutable. This application relates to particular resins and coating compositions formulated with those resins.
A majority of the coating resins commercially used today, are insoluble in water. Therefore, in general practice they have been dissolved in a suitable organic solvent or dispersed in water with the aid of emulsifying or surfactant agents in order to provide a coating composition suitable for application. A serious disadvantage of organic solvent solutions is their inherent potential toxicity, flammability, and environmental pollution.
Many state implementation plans under the Clean Air Act of 1970 have followed the general framework of Rule 66, which was adopted by Los Angeles County, Calif. in 1966 and which greatly limits the quantities of hydrocarbons and photochemical oxidants which may be emitted into the atmosphere.
These regulations are generally addressed to the users of coatings rather than the manufacturer. Coatings manufacture can in general be conducted with little or no emission of solvents into the atmosphere, but the application and curing of these coatings on finished products such as automobiles and refrigerators require the evaporation of large amounts of solvent and thus fall under these regulations.
The most common systems being developed by coatings companies to meet these regulations are waterborne, powder, electrodeposition, high solids, conforming solvents, and polymer-monomer blends for ultraviolet light cure type of coatings. Although very few waterborne coatings are devoid of organic solvents, several of the state regulations have exempted water-thinned coatings which contain 20 percent or less volatile nonexempt material.
Aqueous dispersion or latex-type coatings contain high molecular weight polymers dispersed as an emulsion in an aqueous medium. Generally, acrylic latex coatings exhibit excellent toughness, good chemical and water resistance and excellent durability. However, the use in industrial coatings has been limited because of certain disadvantages (external thickener required for viscosity control, poor pigment dispersability, low gloss, foaming, complex formulation, and substrate wetting problems). Also, such water dispersions, when used to prepare coatings, generally require additional hardeners to overcome residual emulsifiers and curatives to crosslink the resin where water insoluble coatings are desired. Otherwise, such coatings prepared from a water dispersion would simply bleed away with a water wash.
Water reducible polymers contain polar groups, which impart a degree of solubility. They are hybrids of emulsion polymers and solution polymers since they possess properties characteristic of both types. The affinity of the resin for water is controlled by the extent of solvation of the polar groups (carboxylic acid moieties in the case of acrylic polymers) in the polymer backbone. Solvation is achieved by pH adjustment and/or the addition of a water miscible polar cosolvent. Water reducible resins can be alkali soluble (containing acidic groups), acid soluble (containing basic groups), or nonionic (containing amide or hydroxyl groups in conjunction with either acid or basic components). The resins of this invention are of the alkali-soluble type.
Coatings made from the water-reducible resins have advantages over emulsion type coatings. They have better flow and leveling characteristics. Pigment dispersibility is easier. Formulation of the coating composition is not as complex, and gloss of the final coating is higher. Another important benefit of the water reducible type of coating is that it may be formed without substantial amounts of surfactant or emulsifying agents, which in large amounts can inhibit the physical properties of the final coating.
Representative of the types of resins presently utilized in water reducible coatings are: alkyd, styrene acrylics, acrylics, and polyesters. This invention is concerned with the acrylic type of resin. Acrylic water reducible resins having both hard and soft segments and to which a plasticizer is added are disclosed in U.S. Pat. No. 4,064,092, of which there are two divisional applications on file (Ser. Nos. 827,196 and 827,037, now allowed, both filed Aug. 23, 1977).
Both the resins and coating formulations described herein represent an improvement over those in U.S. Pat. No. 4,064,092. The coating compositions described in that patent comprise: a resin, a coalescing solvent, plasticizer, a volatile amine, and water in stated proportions and mixed in a certain manner.
The following improvements over U.S. Pat. No. 4,064,092 were found to be desirable:
(1) lower solvent cost; PA0 (2) lower the amount of organic solvent used (20/80 volume ratio of coalescing solvent to water specified at column 9, lines 20-21 of referenced patent) to decrease atmospheric pollution; PA0 (3) lower phase inversion viscosity; the point at which the mixture of resin, amine, defoamer, and organic solvent (to which mixture water is being added) changes from a water-in-oil dispersion to an oil-in-water dispersion. This would make mixing easier and would require less energy to make up the composition; PA0 (4) increase the solids content of the composition, to give coatings offering improved protection to substrate surfaces, without at the same time thickening the material and making it difficult to apply; PA0 (5) decrease plasticizer content because in some applications plasticizer can be detrimental to the coating, being extractable from the coatings by solvents; PA0 (6) decrease carboxyl group content of the resin to decrease further the water sensitivity of the final coating. PA0 A. a resin which consists essentially of, based on 100 weight percent of monomeric units within the resin PA0 B. a water soluble solvent selected from ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, diethylene glycol monoethyl ether acetate, diethylene glycol diethyl ether, ethylene glycol monomethyl ether acetate, dipropylene-glycol methyl ether, methyl ethyl ketone, propylene glycol ethyl ether, propylene glycol isopropyl ether, propylene glycol butyl ether, acetone, methyl propyl ketone and diacetone alcohol; PA0 C. a plasticizer having a melting point of about -40.degree. C. to about 25.degree. C., a boiling point of at least 95.degree. C. and a solubility parameter of about 8 to about 16; PA0 D. a volatile amine in sufficient amount to result in a pH of from 8 to 14 for the composition; PA0 E. water in a volume ratio to the other volatile components of at least 80:20; PA0 F. defoamer; the improvement which comprises those compositions:
In attacking the above problems, a significant technique has been discovered for controlling the rheology of water reducible compositions.